<!DOCTYPE html>
<html lang="cn">
<head>
    <meta charset="UTF-8">
    <title>JS - canvas 国旗</title>
</head>
<body>

    <canvas id="canvas" width="600" height="400"></canvas>

    <script type="text/javascript">
        // 使用HTML5绘制标准五星红旗
        var canvas = document.getElementById("canvas");
        var context = canvas.getContext('2d');
        var width = canvas.width;
        var height = width * 2 / 3;
        var w = width / 30;//小网格的宽   
        context.fillStyle = "red";
        context.fillRect(0, 0, width, height);
        var maxR = 0.15, minR = 0.05;//   
        var maxX = 0.25, maxY = 0.25;//大五星的位置   
        var minX = [0.50, 0.60, 0.60, 0.50];
        var minY = [0.10, 0.20, 0.35, 0.45];
        // 画大 ☆   
        var ox = height * maxX, oy = height * maxY;
        create5star(context, ox, oy, height * maxR, "#ff0", 0);//绘制五角星
        // 画小 ★   
        for (var idx = 0; idx < 4; idx++) {
            var sx = minX[idx] * height, sy = minY[idx] * height;
            var theta = Math.atan((oy - sy) / (ox - sx));
            create5star(context, sx, sy, height * minR, "#ff0", -Math.PI / 2 + theta);
        }

        //绘制五角星
        /**   
         * 创建一个五角星形状. 该五角星的中心坐标为(sx,sy),中心到顶点的距离为radius,rotate=0时一个顶点在对称轴上   
         * rotate:绕对称轴旋转rotate弧度
         */
        function create5star(context, sx, sy, radius, color, rotato){
            context.save();
            context.fillStyle = color;
            context.translate(sx, sy);//移动坐标原点
            context.rotate(Math.PI + rotato);//旋转
            context.beginPath();//创建路径
            var x = Math.sin(0);
            var y = Math.cos(0);
            var dig = Math.PI / 5 * 4;
            for (var i = 0; i < 5; i++) {//画五角星的五条边
                var x = Math.sin(i * dig);
                var y = Math.cos(i * dig);
                context.lineTo(x * radius, y * radius);
            }   
            context.closePath();
            context.stroke();
            context.fill();
            context.restore();
        }

        //辅助线   
        context.moveTo(0, height / 2)   
        context.lineTo(width, height / 2);   
        context.stroke();   
        context.moveTo(width / 2, 0);   
        context.lineTo(width / 2, height);   
        context.stroke();
        
        //画网格，竖线   
        for (var j = 0; j < 15; j++) {   
            context.moveTo(j * w, 0);   
            context.lineTo(j * w, height / 2);   
            context.stroke();   
        }   
        //画网格，横线   
        for (var j = 0; j < 10; j++) {   
            context.moveTo(0, j * w);   
            context.lineTo(width / 2, j * w);   
            context.stroke();   
        }
        
        //画大圆   
        context.beginPath();   
        context.arc(ox, oy, maxR * height, 0, Math.PI * 2, false);   
        context.closePath();   
        context.stroke();
        
        // 画小圆   
        for (var idx = 0; idx < 4; idx++) {   
            context.beginPath();   
            var sx = minX[idx] * height, sy = minY[idx] * height;   
            context.arc(sx, sy, height * minR, 0, Math.PI * 2, false);   
            context.closePath();   
            context.stroke();   
        }
        
        //大圆中心与小圆中心连接线   
        for (var idx = 0; idx < 4; idx++) {   
            context.moveTo(ox, oy);   
            var sx = minX[idx] * height, sy = minY[idx] * height;   
            context.lineTo(sx, sy);   
            context.stroke();   
        }
    </script>

</body>
</html>